1. Field of the Invention
The present invention relates to a radio frequency identification (RFID) tag.
2. Description of the Related Art
Conventionally, at a frequency, for example, 13 megahertz (MHz) band, used for a radio tag, an antenna has a predetermined impedance, for example, 50 ohms (Ω), and it is relatively easy to be matched with an integrated circuit (IC). A coil antenna is used in such radio tag, to transmit and receive information by electromagnetic induction between a transmitter/receiver that is positioned at a short distance from the radio tag.
As a technique for adjusting a resonant frequency in a low-frequency band, an IC card is known in which a resonant circuit is formed with a coil antenna and a patterned capacitor. Capacitance of a capacitor is adjusted by cutting the capacitor pattern, thereby adjusting the resonant frequency (for example, Japanese Patent Laid-Open Publication No. 2000-235635).
Moreover, a non-contact type IC card is known in which a resonant circuit is formed with a coil antenna and an adjustment resistor. A resistance of the adjustment resistor is adjusted by cutting the adjustment register. By adjusting the adjustment resistor, sharpness of resonance Q of the resonant circuit can be adjusted. An adjustment capacitor may be provided, and by cutting the adjustment capacitor, resonant frequency f can be also adjusted (for example, Japanese Patent Laid-Open Publication No. 2001-10264).
In an RFID tag using a high frequency, a radio wave is transmitted and received between a dipole antenna and an antenna in a transmitter/receiver positioned at relatively long distance from the RFID tag. In such RFID tag, because impedance of an IC varies, a different design from a conventional design is required to match with the antenna. The RFID tag is likely to change characteristics thereof depending on use environments, such as a used frequency, an area, and a material to which the RFID tag is attached.
As a technique of matching an antenna and an IC in a high-frequency band, an antenna having two loading bars formed in a straight line in a dipole form and a stub is used. Since an antenna pattern itself has an inductance component, it is possible to change the impedance characteristic by selecting a loading bar and by cutting a stub being an adjustment part (for example, U.S. Pat. No. 6,028,564).
According to the above conventional techniques, a resonant frequency can be adjusted by cutting the capacitor pattern or the adjustment capacitor, or by cutting the antenna pattern. However, how much adjustment can be achieved when how much these cutting parts are cut is unclear.
Due to this, effectiveness of adjustment cannot be grasped until an adjustment operation, such as cutting of the pattern, is actually performed. Therefore, it takes a long time for the adjustment. In addition, unnecessary operations can be repeated in trial and error, and efficiency in the adjustment operation cannot be improved and cost for the adjustment increases.
Since an RFID tag uses high-frequency band (900 MHz band, or 2.45 Gigahertz (GHz) band), match between an antenna and an IC is particularly important.
The techniques disclosed in Japanese Patent Laid-Open Publication No. 2000-235635 and Japanese Patent Laid-Open Publication No. 2001-10264 are for a low-frequency band (13 MHz band) in which information is transmitted and received by electromagnetic induction using a coil antenna with a transmitter/receiver positioned at a relatively short distance from the radio tag. In the RFID tag using a high frequency band, a dipole antenna is used, therefore, the techniques for a low-frequency band cannot be applied.
A technique disclosed in U.S. Pat. No. 6,028,564 is for a high-frequency band in which a dipole antenna is used. However, how a loading bar is selected, and how much adjustment can be achieved when how much of the stub is cut are unclear.